Include the `where` clause from a "real" type when mapping type aliases

Include the `where` clause from a "real" type when mapping type aliases

ie if we have a
```
struct A<T: Deref> where T::Target: Trait {}
pub type B<T> = A<T>;
```

this includes the `where` clause so that we end up calling
`writeln_opaque` for `struct B<T: Deref> where T::Target: Trait {}`
instead of `struct B<T: Deref> {}`.